It is known in the art that plants can be grown from seeds or seedlings or layers of these, which are contained in plant holders, that are inserted together with the plants in their final locations in the ground, e.g. in open areas. It is important that the plant holder have a certain ruggedness and compactness, so that it does not fall apart in transport and handling. On the other hand, with excessive compression of the substrate consisting of peat, humus, and nutrients, a plant holder with such great density is obtained that the plants cannot develop properly. A process for the construction of plant holders is disclosed in Austrian Pat. No. 294,484, in which a substrate mixture of peat, humus, and nutrients is precompressed into a plant holder. The holder is inserted into a form so that there is a space between the holder and the wall of the form, in which a soft, elastic plastic foam material, e.g. polyurethane foam, is produced. The polyurethane foam encloses and holds together the plant holder in the manner of a pot after removal from the form. Furthermore, from Swiss Pat. No. 514,980, as well as from German Pat. No. 2,025,316, a process is known, in which the liquid foam components are added as a binding agent to the mixture of peat, humus, loam, or clay and nutrients.
The resulting mixture is inserted in a form, where the binding agent is allowed to set. Here, too, polyurethane components that foam up in the presence of water have proven themselves as appropirate bindng agents, where polyether-based components are used as the polyol components, and the toluoldiisocyanates, known from the manufacture of polyurethanes, are used as the isocyanate components. Th direct use of synthetic foam materials as a substrate for a soilless plant culture is also well known in the art. Thus, German Pat. No. 1,018,077 discloses a synthetic-plastic foam, consisting of aminoplastic, phenoplastic, and melamine plastic, used as the culture medium for soilless plant growth. Furthermore, in German Pat. No. 1,221,484 a process for the construction of plant holders is disclosed, in which particles of an elastic plastic, e.g. polystyrol, are mixed with inorganic, inelastic materials, such as pumice, by means of a bindng agent, such as rubber latex. To this mixture of particles, such additional organic substances as coffee grounds, etc., or replacements loaded with fertilizing salts may be added to the binding agents prior to permitting setting to take place. Soft polyurethane foams themselves have also become known for soilless plant cultivation. Thus, German Pat. No. 2,063,715 discloses the use of a dark foam material obtained through the addition of soot or an equivalent blackcoloring material. This material can then be used in the form of cubes or similar shapes as a plant holder, which stores solar heat particularly well. The soft polyurethane foam materials in general are open-celled, which enables them to accept water, making them suitable as material for plant holders. However, with use of a polyurethane foam block that is too soft, the roots of the plant, as soon as they have reached a certain height, are no longer held sufficiently strongly in the plant bed, so that the plant can easily sway. This is particularly disadvantageous in outdoor planting, because then the most delicate roots at the periphery of the root bundle, which have developed and extended into the foam material, can tear out or off while the plant sways in a wind or a storm. Therefore, it is essential that the foam have a greater ruggedness and rigidity. But the consequence of this is that when the roots grow into the pores of the foam, the latter offers a pronounced permanently-elastic resistance, which can inhibit the growth of the roots.
Now, it has been determined that a substrate consisting of 30 -- 70 percent by volume of very finely pulverized polyurethane foam flakes with a flake or particle size of 3 - 15 m, preferably 5 - 8 mm, and 70 - 30 percent by volume of peat, humus, or loam and fertilizers if required. The particles of these are held together by an elastic binding agent, having particularly favorable properties for plant holders. For the mixture, according to the invention, a polyurethane foam, which has been made recticular by chemical or physical processing, as described in German Pat. No. 1,036,510, or German Pat. Nos. 1,159,164 and 1,179,362, is particularly appropriate. Through this processing, a particularly loose, grid-like structure of the foam is obtained, which, in combination with peat and the other substrate particles, gives the plant holder a particularly loose and yet rugged structure with good water-retaining properties for the purposes of the invention. Particularly advantageous, here, is the use of a soft polyurethane foam, which, by means of physical processing, has been changed into the desired grid-like structure for the purpose of breaking open the cell walls, as is described in detail in German Pat. No. 1,504,096 which corresponds to U.S. Pat. Nos. 3,175,030 3,175,025; and 3,329,759. In processing the foam materials, the pores that are still partially closed are opened, and at the same time the thickness of the foam skeleton is reduced. The foam thus obtained, with a reticular structure, lends the substrate material a particularly high capillary capability for air and water.